The invention relates to a receiver which is tunable to any one of a plurality of channels by means of channel-select commands. The invention may be applied, for example, in a mobile phone that operates in accordance with a standard dividing a frequency band into various channels.
U.S. Pat. No. 5,551,235 describes a receiver which is part of a cellular telephone transceiver including a transmitter. The receiver comprises a local oscillator which, in a reduction to practice, was varied from 1031 to 1064 MHz in 25 kHz increments to effect scanning of base stations with channels having a 25 kHz bandwidth.
A synthesizer controls the frequency of the local oscillator. The synthesizer includes a reference oscillator. A divide-by-M divider is connected to the output of the reference oscillator and a divide-by-N divider is connected to the output of the local oscillator. The reference frequency is divided down by M, and the local oscillator frequency is divided down by N, both N and M being programmable such that they may be varied. A frequency comparator compares the outputs of the dividers. The result of the comparison is an error signal, which drives a tuning port of the local oscillator. To adjust the local oscillator frequency, the divide-by-N divider is varied until its output frequency is substantially equal to the output frequency of the divide-by-M divider.
The reference oscillator is the reference for the frequency generation system, and is, for example, a 2.5 ppm (parts per million) oscillator. If the reference oscillator is off by 2.5 ppm, then the local oscillator will be off by 2.5 ppm. By reducing the bandwidth of IF filters and/or other filters, the tolerance of the reference oscillator can be significantly loosened to 10 ppm.
It is an object of the invention to allow a reduction of cost.
The invention takes the following aspects into consideration. A receiver which is tunable to any one of a plurality of channels by means of channel-select commands, should preferably provide the following certainty. It is certain which channel-select command has to be applied for tuning the receiver to a particular channel. If the receiver does not provide this certainty, it will generally be necessary to apply various channel-select commands until the receiver is tuned to a desired channel. This is time-consuming. Moreover, it will be necessary to detect that the receiver is tuned to the desired channel. This may also be time-consuming or, what is worse, may even not be possible for one or more channels.
The prior-art receiver provides the aforementioned certainty. The channel-select commands can be thought of as various values for the N of the divide-by-N divider. The synthesizer, in effect, translates a value for the N of the divide-by-N divider into a local oscillator frequency determining to which channel the receiver is tuned. The local oscillator frequency has an inaccuracy which is substantially equal to that of the reference oscillator. It has been mentioned that the inaccuracy of the reference oscillator is loosened from 2.5 to 10 ppm while, in a reduction to practice, the frequency of a local oscillator was varied from 1031 to 1064 MHz in 25 kHz increments. This is done to effect a scanning of base stations with channels having a 25 kHz bandwidth. A 10 ppm inaccuracy of a 1000 MHz frequency is equal to an inaccuracy of 10 kHz. Consequently, the inaccuracy of the local oscillator frequency is within the bandwidth of the channels. Thus, it is certain which value for the N of the divide-by-N divider has to be applied for tuning the prior-art receiver to a particular channel.
According to the invention, a receiver which is tunable to any one of a plurality of channels by means of channel-select commands, carries out the following steps. The receiver tunes to a channel with a signal having a channel identification that identifies the channel. The receiver defines a tuning reference on the basis of the channel identification and the channel-select command that is used for tuning to such a channel.
The aforementioned steps can provide the certainty which channel-select command has to be applied for tuning the receiver to a particular channel. Consequently, according to the invention, the receiver need not rely on accuracy of tuning components, such as a reference oscillator, for example, to provide this certainty. Thus, the invention allows the use of low-accuracy tuning components, which are generally cheaper than high-accuracy tuning components. Consequently, the invention allows a reduction of cost.
The invention and additional features, which may optionally be used to implement the invention to advantage, are apparent from and elucidated with reference to the drawings described hereinafter by way of non-limitative example.